I expect that people making the "monopoly" claim are using imprecise language that may vary in precision based on jurisdictional definitions of monopolies, and

I expect that people making the "monopoly" claim are using imprecise language that may vary in precision based on jurisdictional definitions of monopolies, and fb's market share in any given jurisdiction. Laws about monopolies and antitrust exist to prevent or mitigate certain problems that arise when companies have disproportionate power and influence on society. Whether or not fb qualifies as "monopoly", there are many cogent arguments to be made that about its negative effects due to its disproportionate power.